Brushless electric motors are well known in the art and are characterized by internal electric circuitry which provides electrical commutation. Brushless direct current (D.C.) motors are routinely produced in high volume for use in a multitude of industrial applications such as fan motors. As a result, it is imperative that the design of the motor be simple, and that it can be adapted to low cost automated manufacture and assembly techniques.
Known brushless direct current motors include the electric fan motor No. TA 300 D.C. manufactured by the NIDEC Corporation of Torrington, Connecticut. This motor includes a rotor and a housing. Concentric with the rotor is a stator and a printed circuit board, both mounted within the housing. On the printed circuit board are electrical components which comprise a commutating circuit. The circuit contains a sensor, typically a Hall cell, which is used as a trigger. Such sensors are characterized by a limited range, and therefore must be carefully located with respect to the stator and the rotor.
The precision with which the sensor must be positioned has limited the extent to which the motor assembly can be automated. Known motors are assembled from discrete subassemblies, with the sensor located on one subassembly (a printed circuit board), while the stator comprises another. Production tolerances of known subassemblies will, when configured together, often produce a stator-sensor relative position which will be out of the sensor's range in an unacceptable number of motors.
For example, while stators of a known design can be snapped onto a bearing tower assembly within the housing, tolerances in stator magnet laminations must be considered in the design of the snap engagement mechanism. As a result of the clearance needed for stator magnet laminations, the stator can be located with respect to the bearing tower only with limited precision. The printed circuit board containing the sensor is fixed separately to the housing, and will then have an unacceptably wide range of vertical and angular positions with respect to the stator. Since the printed circuit board is two dimensional and lacks any intrinsic means for fixturing, it must be manually affixed within the housing at a position which will ensure that the sensor is located within its range. The absence of any self-fixturing mechanism with the printed circuit board also precludes the use of automated assembly techniques with respect to the printed circuit board, and ultimately limits the adoption of automated assembly techniques with respect to the other motor components.
To accomplish precise sensor location, known brushless electric motors using a conventional two dimensional printed circuit board require separate positioning of the magnetic sensor out from the plane of the printed circuit board. These motors are characterized by a separate pedestal or an equivalent that must be attached to the printed circuit board. A separate pedestal offers only marginal improvement in the accuracy of sensor location and adds another labor intensive step to the motor assembly process.
It would be advantageous to have a method of assembling an electric motor characterized by a printed circuit board which allows for self-fixturing and simplified assembly with other motor components, and which further would provide for a precisely located sensor. The present invention is drawn towards such a method.